Abnormal immunologic reactions, both humoral and cellular, have been observed in the rheumatic diseases. We are trying to understand how these abnormalities related to the development and/or progression of rheumatoid arthritis and systemic lupus erythematosus. One mechanism is tissue injury by immune complexes. Cryoprecipitable proteins appear to be analogues of circulating immune complexes. Within these cryoproteins are antigens and antibody. Characterization of these immunoreactants with particular reference to antigens unique to these diseases continues to be the emphasis of this research.